A common problem in reciprocating gas compressors is how to reduce (or change) volumetric flow rates from normal (full capacity) to a lower rate. Most gas compressors use plate or reed valves where the pressure drop across the valve acts to open the valve. There is normally a spring to help ensure that the valve returns to the closed position. However, because there is a spring, these valves are highly susceptible to hitting resonant conditions which can end up destroying the valve in a short period of time. One of the options for reducing volumetric flow rates is to use a variable speed drive so that the number of cycles per second is varied in line with demand. Unfortunately the variable speed also greatly increases the likelihood that a valve will suffer from resonance issues at certain speeds (frequencies). In addition, large variable speed drives are generally more complicated (and hence more expensive) than fixed speed drives.
The valves in gas compressors are normally passive devices and it has been an object of much study to improve them so they can be actively controlled. One solution that has been developed uses a hydraulically powered plunger to hold the inlet (low pressure—LP) valve open for part of the discharge stroke, so that some of the air drawn in through the low pressure inlet is blown back out of the cylinder. In this way it is possible to vary the volumetric flow rate.
In a combustion engine, the valves are actively controlled, for example, by a cam shaft and timing chain. This means that these types of valves are also suitable for acting in both gas compressors and gas expanders, where the timing of the valves must be varied. This can be carried out by changing the camshaft timing for example using hydraulic phasers.
In accordance with a second aspect, there is provided an apparatus for compressing and/or expanding a gas comprising a positive displacement device having a space forming a working volume for compressing or expanding the gas between a lower pressure LP region and a higher pressure HP region to which it is respectively connected via at least one LP valve and via at least one HP valve, the apparatus further comprising a control system for actuating the HP and LP valves, wherein the control system is configured to run an operating mode of the apparatus in which, during at least one cycle, there is either a net gas flow from the LP to HP region, or, a net gas flow from the HP to LP region, as well as bidirectional flow of gas through both the at least one HP valve and at least one LP valve in that mode of operation. By “bidirectional flow” is meant that within one cycle, the flow through a valve goes through in one direction and then reverses to go through in the opposite direction (as opposed to split flow simultaneously in both directions through a valve).
Hence, in accordance with the invention, the apparatus is configured with a (e.g. pre-programmed) mode of operation involving bidirectional flow through one of the valves. Valve closure settings that involve this bidirectional flow will usually be calculated using a relationship that links the respective LP and HP valve settings as matched pairs. Where the % compression (or expansion) power is being modulated, these bidirectional valve settings may be used as stepping stones to go between other more thermodynamically desirable or mechanically optimised valve timing paths (which may not involve pairs of settings with bidirectional flow).
Cycle means a full reciprocation from TDC to BDC and back to TDC.
100% compression flow rate means the maximum volumetric flow of LP gas compressed through apparatus per cycle. 100% expansion flow rate means the maximum volumetric flow of HP gas that has been expanded through apparatus per cycle.
In one embodiment, the amount of bidirectional flow (i.e. by which is meant the actual amount of reverse flow) through each of the at least one HP valve and at least one LP valve exceeds 5% (or even 10%) either of the 100% compression flow rate and/or of the 100% expansion flow rate.
In one embodiment, the bidirectional flow through one of the at least one HP and LP valve changes to unidirectional flow in other cycles of that mode of operation.
In one embodiment, the flow through both the at least one HP and LP valve is unidirectional in other cycles of that mode of operation.
In accordance with a second aspect, there is further provided an apparatus for compressing and/or expanding a gas comprising a positive displacement device having a space forming a working volume for compressing or expanding the gas between a lower pressure LP region and a higher pressure HP region to which it is respectively connected via at least one LP valve and via at least one HP valve, the apparatus further comprising a control system for actuating the HP and LP valves, wherein the control system is configured to run an operating mode of the apparatus that implements an algorithm using the relationship b=K a(Z/Y)+C that links the timing of every HP closure event to a LP valve closure event, whereby a, b, Y and Z are as identified according to FIG. 12 and K and C are constants of proportionality, in order to determine the LP and/or HP valve closure events for that operating mode. The constants of proportionality will vary for different respective types of systems. For example it may depend upon the amount of dead volume and/or the pressure ratio between HP and LP regions and/or any pressure drop through valves. For a system where the dead volume is minimal, the pressure ratio is modest and the pressure loss through valves is low, K will tend to 1 and C will tend to zero, in other words b will tend to equal a(Z/Y).
Applicant is first to appreciate the control logic that the LP and HP valve timings for a particular % flow rate are related in that they are a scaled mirror image of each other about Path 2, as FIG. 10. That is, by knowing the HP valve timing and the desired % flow rate, the LP valve timing could be determined using a scale rule about Path 2.
In one operating mode, either b or a is determined for a chosen a or b value, respectively, to determine the timing of a valve closure event using the relationship b=Ka(Z/Y)+C.
In one embodiment, the operating mode involves variation of the flow rate over a series of cycles from a first selected gas flow rate to a second selected gas flow rate whereby each value lies anywhere between a 0 and 100% HP region to LP region (expansion type) flow rate and/or a 0 and 100% LP region to HP (compression type) region flow rate and wherein a combined LP and HP valve timing route is determined using the relationship b=Ka(Z/Y)+C.
In one embodiment, the operating mode comprises at least one cycle in which there is either a net gas flow from the LP to HP region or a net gas flow from the HP to LP region, and there is also bidirectional flow of gas through both the at least one HP valve and at least one LP valve during that cycle.
In accordance with a second aspect, there is further provided an apparatus for compressing and/or expanding a gas comprising a positive displacement device having a space forming a working volume for compressing or expanding the gas between a lower pressure LP region and a higher pressure HP region to which it is respectively connected via at least one LP valve and via at least one HP valve, the apparatus further comprising a control system for actuating the HP and LP valves, wherein the control system is configured to run an operating mode of the apparatus in which there is variation of the flow rate from one value to another value both lying between 100% compression flow rate and 100% expansion flow rate per cycle and both LP and HP valve timings are changing between at least some adjacent cycles.
Applicant is first to appreciate that variation of flow rate through a series of unloaded states may be carried out by changing both the HP and LP valve timings.
In accordance with a second aspect, there is further provided an apparatus for compressing and expanding a gas comprising a positive displacement device having a space forming a working volume for compressing or expanding a gas between a low pressure region and a high pressure region to which it is respectively connected via at least one LP valve and via at least one HP valve, the apparatus further comprising a control system for actuating the HP and LP valves, wherein the control system is configured to run an operating mode of the apparatus in which flow rate gradually changes such that the function of the working volume changes from compression to expansion, or vice versa, over a series of cycles (in a series of steps that could be graduated or continuous) by changing the timing of the respective HP and LP valve closure events.
Applicant is first to appreciate that rather than switching immediately from a compression setting to an expansion setting, this can be achieved as a gradual alteration of flow rate using HP and LP valve closure events.
For example, the control system may gradually change the function of the working volume from 80% compression flow rate to 80% expansion flow rate, either continuously or, for example, in steps of 5%. The change may happen over 1, 3 cycles or 10 or 50 or 100 cycles.
In one embodiment, the operating mode includes at least one cycle in which a LP and HP paired valve combination lies inside a region bounded by Paths 1 and 3, as shown in FIG. 10.
In one embodiment, the operating mode includes at least one cycle in which a LP and HP paired valve combination lies along Path 2 and where flow rate is less than 100% compression flow rate and less than 100% expansion flow rate, as shown in FIG. 10.
In one embodiment, the operating mode includes following a particular LP valve closure timing path to vary flow rate (using partially unloaded states) between respective cycles that is linked to an associated matched HP valve closure timing path.
In one embodiment, any pair of matched LP and HP valve closure timing paths in an operating mode are each scaled mirror images as shown in FIG. 12.
In one embodiment, the operating mode involves variation of the flow rate from one value to another value both lying between 0% compression flow rate and 100% compression flow rate per cycle. Hence, the amount of compression may be modulated.
In one embodiment, the operating mode involves variation of the flow rate from one value to another value both lying between 0% expansion flow rate and 100% expansion flow rate per cycle. Hence, the amount of expansion may be modulated.
In one embodiment, the operating mode involves variation of the flow rate from one value to another value both lying anywhere within the total range defined by 100% compression flow rate and 100% expansion flow rate per cycle. Moreover, the function of the working volume could change from compression to expansion and vice versa.
In one embodiment, the gas flow rate is varied in a continuous or stepwise manner.
In one embodiment, the positive displacement device is a linear device and is preferably a reciprocating piston assembly. The valves are preferably laterally reciprocating valves. Ideally, the valves are laterally and linearly reciprocating, multi-apertured screen valves.
In one embodiment, the control system is configured only to control the timing of the LP and HP valve closure events.
As will be appreciated from above, where the positive displacement device (e.g. a half-engine) needs to function alternately as both a compressor and expander (e.g. in a thermodynamic system), the second aspect allows its function to switch by gradually changing flow rate through the device over a series of cycles from a chosen % compression power to a selected % expansion power (or vice versa) by changing HP and LP valve closure events.
The device is preferably configured such that the HP and/or LP valves open either when there is minimal gas in the working volume or when the pressure across the valve is at or near pressure equalisation. Ideally, the device is configured such that the HP and/or LP valves open automatically at or near pressure equalization. If the valve is required to open when there is not pressure equalisation this could be done with the use of a poppet valve and associated cam shaft/actuator to open the valve against any pressure difference. This is normally not a preferred embodiment as this opening will result in an energy loss unless the amount of working volume is minimal at this point, for example only the dead volume at TDC. Advantageously, the device is configured such that a valve closure signal has no effect when a valve is already closed.
As indicated earlier in relation to the 1st aspect, Applicant's earlier application, WO2009074800, describes a lightweight sliding screen valve comprising a flexible multi-apertured valve plate configured for lateral reciprocation, which can conform to the face of a multi-apertured valve seat due to its flexibility and hence provide a good quality seal in response to a pressure differential across the valve, and also lock in the closed configuration in response to the pressure differential. It is designed to open automatically upon pressure equalization and is designed to open and close quickly, which makes it suitable for use in a half-engine of a PHES system and in a half-engine where gas mass flow rates are preferably only controlled by valve closure timing events, as described in relation to the first aspect.
In an embodiment (a) where only valve closure events are controlled, the control system is configured to decrease (or respectively increase) net mass flow through a half-engine acting as an expander by advancing (resp retarding) the closure of the high pressure (inlet) valve on the downstroke, optionally whilst using almost the full exhaust stroke for exhaust.
In an embodiment (b) where only valve closure events are controlled, the control system is configured to decrease (or respectively increase) net mass flow through a half-engine acting as an expander by advancing (resp retarding) the closure of the low pressure (exhaust) valve on the upstroke, optionally whilst using almost the full inlet stroke for inward transfer from HP and expansion, resulting in re-compression of gas which had been expanded.
In an embodiment (c) where only valve closure events are controlled, the control system is configured to decrease (resp increase) net mass flow through a half-engine acting as a compressor by retarding (resp advancing) the closure of the low pressure (inlet) valve on the upstroke, optionally whilst using almost the full intake stroke for intake.
In an embodiment (d) where only valve closure events are controlled, the control system is configured to decrease (resp increase) net mass flow through a half-engine acting as a compressor by retarding (resp advancing) the closure of the high pressure (exhaust) valve on the downstroke, optionally whilst using almost the full exhaust stroke for compression and outward transfer to HP, resulting in re-expansion of gas which had been compressed.
In a further embodiment there is contemplated a combination of embodiments a and b directly above.
In a further embodiment there is contemplated a combination of embodiments c and d directly above.
The apparatus may form part of a system for carrying out a gas based thermodynamic cycle, for example, a PHES system, as described in connection with the first aspect above.
There is further provided apparatus for compressing and/or expanding a gas comprising a positive displacement device substantially as hereinbefore described with reference to any of FIG. 4 to 6, or 8 to 12. The apparatus may be pre-programmed to follow a valve timing route involving variation of flow rate and including at least some LP and HP paired valve combinations lying inside a region bounded by and including Paths 1 and 3 and calculated using the relationship b=Ka(Z/Y)+C.
There is further provided a method of operating apparatus as described above, wherein the control system carries out a mode of operation as specified above.